AD8057 datasheet(11/16 Pages) AD

11 / 16 page

REV. B

AD8057/AD8058

–11–

FREQUENCY (Hz)

100

0.1

100M

100

10k

100k

10M

TPC 35. Current Noise vs. Frequency

FREQUENCY (MHz)

100

0.1

1000

100

TPC 36. Output Impedance vs. Frequency

APPLICATIONS

Driving Capacitive Loads

When driving a capacitive load, most op amps will exhibit over-

shoot in their pulse response.

Figure 3 shows the relationship between the capacitive load

that results in 30% overshoot and the closed-loop gain of an

AD8058. It can be seen that, under the Gain = +2 condition,

the device is stable with capacitive loads of up to 69 pF.

In general, to minimize peaking or to ensure device stability for

larger values of capacitive loads, a small series resistor, RS, can

be added between the op amp output and the load capacitor,

CL, as shown in Figure 4.

For the setup shown in Figure 4, the relationship between RS

and CL was empirically derived and is shown in Table I.

CLOSED-LOOP GAIN

500

400

300

200

100

RS = 2.4

RS = 0

Figure 3. Capacitive Load Drive vs. Closed-Loop Gain

Table I. Recommended Value for Resistors RS, RF, RG vs.

Capacitive Load, CL, Which Results in 30% Overshoot

Gain

CL w/RS = 0

Ω

CL w/RS = 2.4

Ω

(

Ω)(Ω)

(pF)

100

104

153

100

33.2

186

270

100

245

500

100

870

1580

–2.5V

50k

VIN = 200mV p-p

AD8058

0.1 F

10 F

0.1 F

10 F

+2.5V

VOUT

FET PROBE

Figure 4. Capacitive Load Drive Circuit

100mV

50ns/DIV

200mV

–100mV

–200mV

+ OVERSHOOT

29.0%

100mV/DIV

Figure 5. Typical Pulse Response with CL = 65 pF,

Gain = +2, and VS =

±2.5 V

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